1. Field of the Invention
This invention relates to a continuously variable transmission and, in particular, to an electronically-controlled belt-type continuously variable transmission.
2. Description of Related Art
A V-belt type continuously variable transmission is widely used on vehicles such as scooter-type two-wheeled vehicles. The V-belt type continuously variable transmission includes a primary sheave disposed on a primary shaft for receiving output of a power source such as an engine, and a secondary sheave disposed on a secondary shaft for the output to be taken out to a drive wheel. A V-belt is routed around the primary and secondary sheaves and continuously changes a transmission ratio between the sheaves by changing groove widths of the sheaves with a groove width regulating mechanism, thereby regulating the round routing radii of the V-belt on the respective sheaves.
The primary and secondary sheaves typically comprise a fixed flange and a movable flange forming a V-groove therebetween. The movable flanges are freely movable in the axial direction on the primary or secondary shaft. The groove width regulating mechanism continuously regulates the transmission ratio by moving the movable flanges.
An electric motor may be used to move the movable flanges. The motion thrust of the electric motor can move the movable flange both to narrow (toward top ratio) and widen (toward low ratio) the primary sheave groove width, thereby freely regulating the groove widths (see, for example, JP-3043061).
Conversely, in a mechanical continuously variable transmission, roller weights disposed inside the primary sheave move outward due to centrifugal force and press and cause the movable flange on the primary sheave side to move. The groove width of each sheave is regulated by the balance between the pressing force of the roller weights of the primary sheave and the force of a spring and a torque cam of the secondary sheave.
Because centrifugal force works on the roller weights along with rotation of the primary sheave caused by engine torque, a belt pressing force on the primary sheave side is produced immediately following an increase in engine revolutions at the time of start. By the pressing force, the belt routed around both sheaves becomes taut and does not flutter due to variations in torque at the time of start.
On the other hand, in an electronic continuously variable transmission, while displacement of the movable flange on the secondary sheave side is effected by belt pressing force of the spring and the torque cam, as in the mechanical continuously variable transmission, displacement of the movable sheave on the primary sheave side is controlled with the electric motor. Until vehicle speed exceeds a specified value, the movable flange is controlled to be held to a low position in which the primary sheave groove width becomes the maximum.
With the above constitution, the electric motor does not work until the vehicle speed reaches a specified value, and the movable flange on the primary sheave side remains fixed to the low position, so that no belt pressing force is produced. Even if the belt on the primary sheave side makes motion in slackening direction, the movable sheave cannot follow the motion. Therefore, the belt might flutter. If the belt flutters at the time of start, the rider may feel uncomfortable because of vibration caused by the flutter, or the flutter may cause an unusual or loud noise.